In the best case, if you open a story and let it play from beginning to end, the story_frames_viewed field is too small by 1.
If you use the frame navigation and close the story from a earlier frame, the story_frames_viewed field is off by a lot.
Basically, this field uses the currentFrame.index prop so it may have been right before the cover page and frame navigation but now it is wrong. We have to keep track of all the frames that have been viewed.